This invention relates to methods and compositions for digesting of asbestos (such as, chrysotile asbestos) into a non-asbestos material while the asbestos is a component of a gypsum-containing cementitious composite, and especially to methods and compositions for the in-place digestion of chrysotile asbestos present in composite material that is bonded to a support structure.
Chrysotile asbestos is a serpentine asbestos fibrous-like material consisting of alternating layers of silica and magnesium oxide/hydroxide bound to each other through covalently shared oxygen.
At least in part because of its availability and unique fire resistance and thermal properties, chrysotile asbestos has been used commercially in a variety of building products, including, for example, fire resistant roofing shingles, acoustical plasters, fire resistant and thermally insulating coating compositions and the like. In the formation of fire resistant coating compositions, found to be appropriate for treatment by the present invention, small amounts of chrysotile asbestos were mixed with a settable bonding material comprising gypsum (e.g., calcium sulfate hemihydrate) and, optionally, other materials such as vermiculite and the like. The composition was then applied to a structure where it forms a hardened coating. For example, such compositions found considerable use in multi-story buildings in which the gypsum containing composition was applied as an adherent coating to steel girders, floor support plates, concrete decking and the like to provide fire resistant and high-temperature thermal insulation properties which aid in preventing damage and even possible collapse of such buildings in the event of fire.
In recent years asbestos has been classified as a regulated material in the United States. Federal, state and local government agencies have promulgated regulations dealing with the use and disposal of asbestos-containing building materials. The U.S. Environmental Protection Agency (xe2x80x9cEPAxe2x80x9d) has defined asbestos-containing material (xe2x80x9cACMxe2x80x9d) as a material which contains greater than one percent (1%) asbestos and requires special handling of such material. In accordance with various regulatory procedures, various safeguards are employed to protect workers from inhaling asbestos fibers during removal or demolition activities. Examples of such safeguards include requiring workers to wear approved respirator or breathing apparatus, as well as protective clothing, requiring any area in a building in which asbestos-containing material is being removed to be isolated or enclosed from the remainder of the building, and requiring the enclosed work area to be kept at a negative pressure by the use of special apparatus equipped with HEPA filters to prevent airborne asbestos fibers from leaving the work area. Such isolation of the work area is an expensive and time-consuming part of the process.
Generally, prior art methods for handling asbestos-containing building materials have taken several approaches. One approach has been to chemically alter asbestos fibers before using them in building products. This approach is discussed, for example, in U.S. Pat. Nos. 4,328,197 and 4,401,636 both to Flowers, and in U.S. Pat. No. 4,474,742 to Graceffa et al.
Graceffa et al. (U.S. Pat. No. 4,474,742) teach treatment of asbestos with hydroxamic acid and iron chelating agents to remove the iron present in the asbestos based on the presumption that the iron is the harmful component. In the Flowers Patents (U.S. Pat. Nos. 4,328,197 and 4,401,636), one is taught to contact asbestos fibers with an aqueous solution of a weak base/strong acid or a strong base/weak acid salt of manganese, chromium, cobalt, iron, copper or aluminum or mixtures thereof, to convert the asbestos fibers into a metal-micelle product. In general, the process contemplated by Flowers is effected by preparing a slurry of asbestos fibers in an aqueous solution of the appropriate salt, effecting the conversion of the asbestos fibers to metal-micelle fibers in the slurry, and recovering the metal-micelle fibers from the slurry for use in the subsequent preparation of the desired fiber-containing end product.
Another approach is to treat previously formed asbestos-containing building materials by encapsulating the materials to thereby prevent the asbestos fibers from becoming airborne. A resinous encapsulating coating material typically would be applied by spraying, brushing or troweling. Care must be taken when using encapsulating methods so as not to physically damage the building material being encapsulated. Encapsulation is a containment method and, thus, the encapsulated asbestos material remains in place during the life of the building.
A number of removal techniques have been proposed, and each has its advantages and disadvantages. For example, it has been proposed to simply scrape or chip away at dry untreated asbestos-containing material and to collect the scraping for discard. This technique, which is referred to as dry removal, is generally considered unacceptable by regulatory authorities since it provides no safeguard against the release of airborne asbestos particles.
Dry vacuum methods have been attempted to overcome the problems of simple dry removal by incorporating an exhaust filtering system to prevent pollution to the outside environment and by using sealed containers for storing and discarding the collected asbestos-containing material. One of the disadvantages of this dry vacuum method is that the bond between the dry building material and the underlying surfaces on which it is coated may be stronger than the vacuum capabilities of the equipment. In those cases, it is necessary to dislodge at least a portion of the asbestos-containing material by scraping or chipping, which has the same limitations as the dry removal process described above.
Wet removal processes have been developed as a means for reducing the problems associated with the various dry removal techniques. Wet removal generally involves wetting a building material with water or water-surfactant solution to soften it and to thereby facilitate its removal. Wet removal clearly represents an improvement over dry removal. However, the use of water as a softening agent is not entirely satisfactory because water penetrates slowly, does not completely wet most building materials, and tends to run off the surfaces being treated.
Over the past several years, wet removal techniques have been improved by devising more effective wetting and/or softening compositions. Recent U.S. patents which relate to such improved wet removal techniques include, for example, U.S. Pat. No. 4,347,150 to Arpin; U.S. Pat. No. 4,693,755 to Erzinger; and U.S. Pat. No. 5,258,562 to Mirick et al.
The Arpin patent discloses a technique for wetting and removing friable insulting materials from an underlying substrate using a two-part wetting system. The first component of the system comprises an aqueous alkali metal silicate dispersion blended with a cationic or nonionic surfactant and the second component comprises a mixture of an acrylic latex and a reagent that is reactive with the alkali metal silicates in the first part. The two parts are stored separately and are mixed shortly before use to form a stripping composition, which facilitates the removal of the building material while encapsulating the individual asbestos fibers contained therein. The removed material must be handled as an asbestos-containing material.
The Erzinger patent exemplifies a wet method for removing asbestos-containing materials from a substrate. This patent discloses applying a composition containing a cellulosic polymer to the asbestos-containing material, allowing the cellulosic polymer-containing composition time to penetrate and wet the asbestos-containing material, removing the wet material from the underlying substrate by mechanical forces, and collecting the removed material for discard.
The Mirick et al. patent is centered on the concept of removing asbestos fiber containing building material by applying a dilute aqueous solution of an acid, which may include a separate source of fluoride ions such as an alkali metal or ammonium salt of hydrofluoric acid to the building material for the purpose of conditioning the material to aid in its removal while partially converting the asbestos fibers. The building material, after having been treated with the dilute acid solution, is preferably removed for further treatment and/or discard. Mirick et al. further contemplate that the wet building material, once removed, can then be digested by immersing the material into a bath of an acid solution, preferably with heating and agitation, until all of the asbestos material has been destroyed.
Several problems are associated with the wet removal techniques. The treatment solutions are conventionally applied to the building material by spray or brush application. These application techniques have an abrasive quality, which may dislodge at least a portion of the surface of the building material causing some asbestos fibers to become airborne. Further, such application can provide delivery of only small amounts of the active materials on a per pass basis. Attempts to apply greater amounts on a per pass application merely causes run-off of the excess over that which the building material is capable of absorbing within the application time. Thus, even attempts to totally wet a material is difficult to achieve and requires, at least, multiple applications of limited amounts. Finally, the conventional means of applying liquid to asbestos-containing materials do not provide a way to control dosage.
More recently, several compositions have been disclosed that are capable of treating asbestos containing building materials so that the treated material transforms into a non-asbestos containing material while being retained as part of the building structure. These compositions are disclosed in U.S. Pat. Nos. 5,753,031; 5,753,033; 5,753,034; 5,753,035; and 5,741,358. The teachings of these patents are incorporated herein in their entirety by reference.
Although the recently disclosed digestion compositions provide a means for converting asbestos containing material to a non-asbestos material while in place, there still remains concern with the mode of application of such compositions to the asbestos-containing material so as not to dislodge the building material being treated causing some asbestos fibers to become airborne. U.S. Pat. Nos. 5,741,358; 5,753,032; and 5,743,841 suggest applying foam compositions made up of the acidic digestion composition and foaming agents having certain ratio of cationic and non-ionic functionality. These foam compositions provide a means of xe2x80x9csoftlyxe2x80x9d applying the digestion composition to the asbestos containing building material. However, they have certain drawbacks, namely, they require the foaming composition to be applied through a multi-phase static aerator while using high pressure apparatus. They may exhibit high degrees of drainage from the foam when using commercially acceptable amounts of foaming agent.
It is desired to have a means of applying a composition capable of transforming chrysotile asbestos containing material to a non-regulated material in an effective and efficient manner.